I think, it didn't start over night, or more precisely after losing to BD in Asia Cup. But yes, cricket is in constant decline. No empire ruins in a day, therefore PAK cricket won't finish in a short span and in between we'll see some sparks in patches like the CT win of the Lord's Test, but as a cricket force it might be tough to return back.
I give an example to explain the situation.
If we take all-time best Indian 19 for a combined Test & ODI squad, almost half of the players will be debuting in this millennium, or say in last 20 years. Kohli, MSD, Ashwin, Sehwag, Zak, UV for sure, may be Rohit, Bumrah, Jadeja, Bhajji, Ishant, BK, Dhawan as well in contention. Add to that Dravid & VVS are missing the cut for 2-3 years. It tells that the game in IND is growing, at least there is a consistency - in every generation quality players are coming & some of them are reaching world class level, few even ATG level. You don't get a Gavaskar, SRT or Kohli in every year - even one in 2 decades is excellent. It's normal for every great sports nation - that's why only 8 teams have won Soccer WC, in every generation they can replace world class players with next batch, and in every WC, they can send a squad realistically capable of winning the cup.
If I do the same for PAK, only 1 player in my 19 will be debuting in last 20 years, that too in 2000. My 19 for a combined team'll be
Mazid, Hanif, Saeed
Zaheer, Javed, MoYo, YK, Mushtaq
Latif, Bari
Imran, A Razzak
Wasim, Waquar, Shoaib, Fazal
Qadir, Saq, Qasim.
This is a symptom of Hungary in soccer - at present, PAK is going through what Hungary went through in between 1960s to 1980s - still they qualified for WC regularly, still they had odd individual brilliance (like beating El Salvador 10-1 in 1982), still they produced few individual talents.... but Hungary was finished as a soccer superpower by early 1960s - today, they are struggling to qualify for a 24 team EURO, when they were regular in WC, from a 6-7 team EURO Zone qualifier. Till 1950s, two of the biggest Soccer rivalry was Hungary V Germany & Hungary V England - today Germany sends their back-ups in friendlies with Hungary & ENG last played Hungary in 2010.
Imran Khan's next 5 years will determine which way PAK Cricket will go - Hungary or Germany - they also came back after 12 years of complete destruction by WW II, to win the WC of 1954, then never looked back (though suffered a small hick-up in 2018, but temporary - Germany should make the 2022 SF at least).